This invention relates to counter type precision aircraft altimeters, and more specifically relates to a novel mechanism for modifying the load dynamics of a counter type altimeter so that a constant load is applied to the aneroid capsule which drives the altimeter.
In a conventional precision altimeter, a pair of evacuated capsules are arranged in parallel with one another and equally drive a common shaft system. A suitable mechanism is provided to change the straight line capsule motion to a rotary shaft motion so that the pressure altitude is linearly related to the rotation of the shaft system. The rotation of the first shaft which is connected to the capsules is usually limited to about 50.degree. of travel for the full range of the altimeter (such as 80,000 feet). The altimeter will then have a pointer which rotates 360.degree. for each 1,000 feet of altitude so that the gear ratio for the mechanism is approximately550 to 1.
An altitude counter is also conventionally provided which generally consists of a three-drum device where the first drum is directly geared to the pointer shaft. Intermittent gears and holding disks, sometimes referred to as Geneva gears, then connect this first drum to the second drum, and connect the second drum to the third drum, in such a manner that, when the drum connected to the pointer moves from 0 to 9, the second and third drums are locked, but when the first drum moves from 9 to 0, the second drum rotates with the first drum for a distance of one digit (36.degree. in this example). Thus, if the first drum is the 100 foot drum, then both the 100 foot and 1,000 foot drum will rotate together for this one digit. Finally, after the first drum has made nine revolutions and the second 9/10ths of a revolution, and as the first drum makes its 10th pass from 9 to 0, the third drum (the 10,000 foot drum) will be picked up with the second drum and all three drums will roll one digit (or 36.degree.) together.
It will be understood that, as the second and third drums are transferred to a new digit position (either moved ahead or back), an additional load is applied to the aneroid capsules which drive the entire mechanism through the very high gear ratio of 550 to 1 in this example. Thus, an offset effect is applied to the capsules which is seen as a slowdown or even a stopping action of the pointer followed by a quickening motion or jump as the capsule accepts the added load. After the extra load of the drum or drums has been removed when the drum or drums have reached their new position, the pointer position may overshoot and come to rest momentarily and then proceed in step with the driving mechanism.